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Abstract:

A system and method for polarimetry are disclosed in which a polarimeter
may include a light source for transmitting a light beam through a sample
within a container; a wavelength selector configured to specify a target
wavelength at which the polarization rotation of the light beam emerging
from the sample will be evaluated; a polarization rotator configured to
be selectively moved into and out of a path of the light beam from the
light source; and a detector for obtaining a first measurement of the
light beam polarization rotation with the polarization rotator outside
the path of the light beam, and a second measurement of the light beam
polarization rotation with the polarization rotator within the path of
the light beam, with both measurements occurring at the wavelength
resulting from the configuration of the wavelength selector.

Claims:

1. A method of polarimetry, comprising: providing a polarimeter including
a light source; transmitting a light from the light source beam through a
sample in a container in the polarimeter; obtaining a first measurement
of polarization rotation of the light beam passing through the sample,
using a target wavelength setting of a wavelength selector in said
polarimeter; inserting a polarization rotator into a path of the light
beam; and obtaining a second measurement of the polarization rotation of
the light beam, after the light beam has passed through both the
polarization rotator and the sample.

2. The method of claim 1 further comprising: calculating a true
wavelength at which the first and second polarization rotation
measurements were obtained from the first and second polarization
rotation measurements and from a value of optical rotary dispersion of
the polarization rotator.

3. The method of 2 further comprising: adjusting the target wavelength
setting, of the wavelength selector, to compensate for a wavelength error
quantity separating the target wavelength and the calculated true
wavelength, thereby canceling an effect of the wavelength error quantity.

4. The method of claim 3 further comprising: conducting a polarimeter
reading of the sample in the container at the adjusted target wavelength
setting.

5. The method of claim 2 further comprising: conducting a polarimeter
reading of the sample in the container using the target wavelength
setting of the wavelength selector; and associating the polarimeter
reading with the calculated true wavelength.

6. The method of claim 2 further comprising: conducting a raw light beam
polarization rotation reading of the sample in the container using the
target wavelength setting for the wavelength selector; and calculating a
corrected reading of the light beam polarization value of the sample from
the raw light beam polarization rotation value and a value of an optical
rotary dispersion of the sample.

7. The method of claim 6 further comprising: applying the light beam
polarization rotation reading correction calculation to subsequent
measurements of samples using said target wavelength setting in said
wavelength selector.

8. A polarimeter comprising: a light source for transmitting a light beam
through a sample within a container; a wavelength selector configured to
specify a target wavelength at which the polarization rotation of the
light beam emerging from the sample will be evaluated; a polarization
rotator configured to be selectively moved into and out of a path of the
light beam from the light source; and a detector for obtaining a first
measurement of the light beam polarization rotation with the polarization
rotator outside the path of the light beam, and a second measurement of
the light beam polarization rotation with the polarization rotator within
the path of the light beam, with both said measurements occurring at the
wavelength resulting from the configuration of the wavelength selector.

9. The polarimeter of claim 8 wherein the polarization rotator is a
crystal quartz rotator.

10. The polarimeter of claim 8 wherein the polarization rotator is
incorporated within a temperature controlled chamber.

11. The polarimeter of claim 10 wherein the temperature controlled
chamber includes a temperature sensor located therein.

12. The polarimeter of claim 10 wherein the temperature controlled
chamber includes at least one of: a heating means; and a cooling means.

13. The polarimeter of claim 8 further comprising: an actuator for moving
the polarization rotator either into or out of the path of the light beam
upon.

14. The polarimeter of claim 13 further comprising: a controller for
providing positioning instructions to the actuator for positioning the
polarization rotator.

15. The polarimeter of claim 8 wherein the wavelength selector comprises:
a monochromator for selecting a target wavelength.

16. The polarimeter of claim 8 wherein the wavelength selector comprises
at least one bandpass filter.

17. The polarimeter of claim 8 wherein the polarimeter is operable to
calculate a true wavelength at which the first and second polarization
rotation measurements were obtained, from the first and second
polarization rotation measurements and from a value of optical rotary
dispersion of the polarization rotator.

18. The polarimeter of claim 17 wherein the polarimeter is further
operable to: calculate a correction to the first obtained measurement of
polarization rotation using the calculated true wavelength.

19. The polarimeter of claim 17 wherein the polarimeter is operable to
adjust the target wavelength setting, in the wavelength selector, to
compensate for a wavelength error quantity separating the target
wavelength and the calculated true wavelength, thereby canceling an
effect of the wavelength error quantity.

20. The method of claim 1 further comprising: identifying an operational
variable within the polarimeter that impacts both the polarization
rotator and the sample; determining a value of an operational variable
active during the second measurement of the polarization rotation from
the difference between the first and second polarization rotation
measurements and from a correlation between the characteristics of the
polarization rotator and values of the operational variable; and
determining an actual value of the operational variable within the
polarimeter active during the first measurement of the polarization
rotation from the value of the operational value determined to be present
during the second measurement of the polarization rotation and a
relationship between the value of polarization rotation of the sample and
the value of the operational variable.

Description:

BACKGROUND OF THE INVENTION

[0001] The present invention relates in general to polarimeters, and more
specifically to polarimeters that are used to measure optically active
fluid samples.

[0002] An existing multiple wavelength polarimeter is shown in FIG. 1. A
broadband light source 1 projects a beam of substantially parallel light
through fixed polarizer 2. The light beam 21, which after passing through
polarizer 2 consists of single-plane polarized components at various
wavelengths, enters Faraday cell 3. Faraday cell 3, typically constructed
of a transparent rod of suitable material arranged axially to the beam,
is wound with a coil carrying an oscillating current transmitted by
signal generator 4. The oscillating current signal causes the plane of
polarization of the parallel-light beam to oscillate about the fixed
direction established by polarizer 2 with an amplitude proportional to
the current in the coil. The light beam 21 then passes through the
optically active sample 24 contained in sample cell 5 (also referred to
herein as container 5). The optical characteristics of the sample 24
generally impart additional rotation of the mean plane of polarization of
the light beam 21 relative to the polarization of the beam established by
fixed polarizer 2. The additional rotation imparted by the beam 21
passing through the sample may be proportional to the concentration of an
optically active constituent in the sample 24 being measured. The term
"optically active sample" is intended to include the case of a null or
blank sample with an optical activity of zero, in addition to real
samples which operate to change one or more characteristics of light beam
21.

[0003] Light beam 21 may then pass through analyzer 6. Analyzer 6 may
include a polarizer mounted so as to be rotatable about the axis of the
beam. The rotational position of the polarizer may be determined by
controller 7 acting through the motor and encoder unit 8. The wavelength
of interest can then be isolated by wavelength selector 9. Wavelength
selector 9 may include a motorized monochromator or filter wheel. The
intensity of the beam 21 arriving at the detector 10 is generally
proportional to the square of the cosine of the angle between the beam
polarization direction upon exit from sample cell 5 and the analyzer
polarization direction.

[0004] Fourier analysis of the beam intensity variation with time
determines the sign of the minimum angle separating (a) the beam
polarization direction upon exit from sample cell 5 from the (b) analyzer
6 polarization direction that is needed in order to null the rotation of
the sample. If this minimum angle is sufficiently small relative to the
amplitude of the oscillating polarization produced by the Faraday cell 3,
then the magnitude of the minimum angle can be determined as well.

[0005] Together, this sign and magnitude information is used to rotate the
analyzer 6 to extinguish or "null" the component of intensity that is due
to the rotation of polarization plane induced by the sample to be
measured. The analyzer 6 angle needed to null the system in this manner,
when no sample is present in sample container 5, becomes the zero
reference. Any additional analyzer 6 angle needed to null the system when
a sample 24 to be measured is present in container 5 constitutes a
measurement of the optical rotation caused by the sample 24. This
additional analyzer 6 angle (i.e. the analyzer angle over and above the
angle needed to null the system when no sample is present in sample cell
5) may be proportional to the concentration of an optically active
constituent in the sample 5 being measured.

[0006] In existing polarimeters of the type depicted in FIG. 1, one source
of measurement error is the wavelength error of the wavelength selector
9. Commercially available compact monochromators intended for use in
benchtop analytical instrumentation typically have a wavelength accuracy
on the order of 1 nanometer (nm) and a wavelength repeatability of plus
or minus 0.2 nm. If wavelength repeatability errors of this magnitude
were present during optical rotation measurements of a normal sucrose
solution at a wavelength of 589 nm at 20° C. (degrees Celsius),
the contribution of this error source to the optical rotation
repeatability would be about 0.025 degrees of rotation. This is much
larger than the 0.002 degree rotation repeatability typically achieved by
fixed-wavelength polarimeters. If the wavelength selector is a wheel or
turret of discrete bandpass filters or if the wavelength is selected with
a manually interchangeable bandpass filters, wavelength errors can also
arise due to the temperature coefficient of the filters, the inclination
of the filter to the beam path, or the degradation of the filter due to
environmental conditions such as humidity or mechanical shock.

[0007] Accordingly, there is a need in the art to address the error that
arises in polarimetry measurements.

SUMMARY OF THE INVENTION

[0008] According to one aspect, the present invention is directed to a
polarimeter that may include a light source for transmitting a light beam
through a sample within a container; a wavelength selector configured to
specify a target wavelength at which the polarization rotation of the
light beam emerging from the sample will be evaluated; a polarization
rotator configured to be selectively moved into and out of a path of the
light beam from the light source; and a detector for obtaining a first
measurement of the light beam polarization rotation with the polarization
rotator outside the path of the light beam, and a second measurement of
the light beam polarization rotation with the polarization rotator within
the path of the light beam, with both measurements occurring at the
wavelength resulting from the configuration of the wavelength selector.

[0009] Other aspects, features, advantages, etc. will become apparent to
one skilled in the art when the description of the preferred embodiments
of the invention herein is taken in conjunction with the accompanying
drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] For the purposes of illustrating the various aspects of the
invention, there are shown in the drawings forms that are presently
preferred, it being understood, however, that the invention is not
limited to the precise arrangements and instrumentalities shown.

[0011] FIG. 1 is a block diagram of a polarimeter;

[0012] FIG. 2 is a block diagram of a polarimeter in accordance with an
embodiment of the present invention; and

[0013]FIG. 3 is a block diagram of a computer system useable in
accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] In the following description, for purposes of explanation, specific
numbers, materials and configurations are set forth in order to provide a
thorough understanding of the invention. It will be apparent, however, to
one having ordinary skill in the art that the invention may be practiced
without these specific details. In some instances, well-known features
may be omitted or simplified so as not to obscure the present invention.
Furthermore, reference in the specification to phrases such as "one
embodiment" or "an embodiment" means that a particular feature, structure
or characteristic described in connection with the embodiment is included
in at least one embodiment of the invention. The appearances of phrases
such as "in one embodiment" or "in an embodiment" in various places in
the specification do not necessarily all refer to the same embodiment.

[0015] The embodiments disclosed herein may be operable to provide
apparatuses and methods for providing multiple wavelength polarimetry
that incorporates the benefits of multiple wavelength measurement without
incurring the wavelength errors that are present in existing wavelength
selectors. An apparatus according to one embodiment of the present
invention is shown in FIG. 2. In addition to the apparatus discussed in
connection with FIG. 1, the apparatus 200 of FIG. 2 may further include
an actuator 12 and/or a temperature controlled chamber 13 which may
include a polarization rotator 11. Chamber 13 may further include
temperature sensor 22 and/or heating/cooling means 23. Heating/cooling
means 23 may include a resistance heater for heating purposes and may
include an active refrigeration system for cooling purposes. However,
other cooling and/or heating systems may be employed, and the invention
is not limited to the heating and cooling apparatuses recited above.

[0016] In this embodiment, a broadband light source 1 may project a beam
of substantially parallel light 21 through fixed polarizer 2. The light
beam 21, which after passing through polarizer 2 preferably includes
single-plane polarized components at various wavelengths, preferably
enters Faraday cell 3. Preferably, Faraday cell 3 causes the plane of
polarization of light beam 21 to oscillate about the fixed direction
established by polarizer 2. Light beam 21 may then pass through
polarization rotator 11 which may be moved into and out of the path of
light beam 21 by actuator 12 in response to a signal from controller 7.

[0017] The polarization rotator 11 may be chosen such that its optical
rotary dispersion, which corresponds to the variation of light-beam
polarization rotation as a function of wavelength, is either (a)
tabulated or (b) characterized by a known function. Polarization rotator
11 may include a planar optical crystal quartz window with the optic axis
of the crystal perpendicular to the faces and parallel to the path of
beam 21. However, polarization rotator 11 is not limited to having the
above-described structure.

[0018] The optical rotary dispersion of such quartz rotators has been well
characterized, for example by the International Commission for Uniform
Methods of Sugar Analysis, as found in the Proceedings of the 22nd
session, 1998, Page 211. Actuator 12 is preferably operable to rotate
polarization rotator 11 into and out of the path of light beam 21. Thus,
actuator 12 is preferably a rotary actuator, and may be powered
electrically, pneumatically, hydraulically, or any combination of two or
more of the foregoing mechanisms for power transmission. Alternatively,
another type of actuator could be used, such as a linear actuator for
moving polarization rotator 11 into and out of the path of light beam 21.

[0019] Because polarization rotation is typically also a function of
temperature, the polarization rotator 11 may be substantially enclosed by
temperature-controlled chamber 13 which may include a temperature sensor
22 and a means for heating and/or cooling 23 (variations of which are
discussed above). If temperature effects on the polarization rotator 11
are well known, the heating and/or cooling means may be eliminated in
favor of using the temperature sensor 22 reading to calculate a
correction to the polarization rotation measured at detector 10. If the
polarization rotator 11 is within the path of light beam 21, polarization
rotator 11 may impart additional rotation of the mean plane of
polarization of light beam 21 relative to the direction established by
fixed polarizer 2. The light beam 21 then preferably passes through the
optically active sample 24 in container 5. The optical characteristics of
sample 24 in container 5 generally impart still further rotation of the
mean plane of polarization of light beam 21.

[0020] As noted elsewhere herein, the additional rotation of the mean
plane of polarization of light beam 21 may be proportional to the
concentration of an optically active constituent in sample 24, in
container 5, to be measured. Light beam 21 may then be transmitted on to
analyzer 6 after passing through sample 24. Wavelength selection may then
be conducted in wavelength selector 9. Measurement of the rotation of the
polarization of light beam 21 may then be conducted using detector 10.
Preferably, wavelength selection and/or measurement of the optical
rotation of the polarization of light beam 21 may be conducted for (a) a
situation in which polarization rotator 11 is not placed in the path of
light beam 21; and (b) a situation in which polarization rotator 11 is
placed in the path of light beam 21.

Methods of Use

[0021] A preferred method for using an embodiment of the above-discussed
apparatus is discussed below. Two consecutive measurements of the optical
rotation of sample 24 may be made at detector 10 to aid in determining
any error in the wavelength setting being used for wavelength selector 9.
In a first measurement, the polarization rotator 11 may be within the
path of beam 21. In a second measurement, the polarization rotator 11 may
be outside of the path of beam 21.

[0022] When the polarization rotator 11 is not within the path of light
beam 21, the measured rotation of the polarization of the beam 21 after
proceeding through sample 24 is as follows:

M 1 = S ( λ ) + δ S δ
λ Δ λ . ( Eq . 1 )
##EQU00001##

[0023] In Equation (1), the first term on the right-hand side is the
rotation of the sample at the desired wavelength, λ (which is also
referred to herein as the target wavelength). The second term on the
right-hand side of equation (1) is the local slope of the optical rotary
dispersion of sample 24 multiplied by the unknown wavelength error,
Δλ.

[0024] When the polarization rotator 11 is moved into the beam path the
measured optical rotation of the polarization of beam 21 increases to:

M2=M1+R(λ+Δλ) (Eq. 2).

[0025] In equation (2), M2 is the measured polarization rotation of
light beam 21 with polarization rotator 11 placed within the path of
light beam 21. The expression λ+Δλ is the wavelength at
which the measurement M2 is taken, which is also referred to herein
as the true wavelength and the correct wavelength. The true wavelength
may differ from the target wavelength desired when configuring wavelength
selector 9 by the extent of the wavelength error present in wavelength
selector 9.

[0026] The term R(λ+Δλ) corresponds to the contribution
of polarization rotator 11 (while in the path of light beam 21) to the
total measured polarization rotation value M2 as measured at
detector 10. The term "R" in equation (2) is the optical rotary
dispersion of polarization rotator 11 which denotes the variation of
polarization rotation as a function of wavelength. As mentioned
previously herein, the optical rotary dispersion may be either tabulated
or characterized by a mathematical function.

[0027] With reference to the mathematical function referred to above, one
exemplary dispersion equation for quartz polarization rotators is shown
in equation (3). We note that the present invention is not limited to
using the equation shown to relate polarization rotation to wavelength.
The specific optical rotation, [α], in degrees of rotation per mm
of quartz at 20° C. with wavelength, λ, in microns is:

[0029] where R-1 is now the inverse of the function which defines the
rotation of the polarization rotator as a function of wavelength.
Consistent with the above discussion of the meaning of "R", the function
R-1 may be applied to the quantity M2-M1 by conducting a
lookup of tabulated data correlating polarization rotation to wavelength.
Alternatively, a mathematical function corresponding to the inverse of
the function denoted by "R" may be applied to the quantity
M2-M1 to yield the true wavelength. Once the correct wavelength
is determined using equation (4), we may now work to correct the value of
the rotation of the polarization of light beam 21, denoted by the symbol
M1. Various options may be available for determining the true light
beam 21 polarization rotation, which are discussed below.

Methods of Correcting Measurements Due to Wavelength Error

[0030] Using a first approach, the measured light beam polarization
rotation may be treated as having occurred at the true wavelength,
λ+Δλ, rather than at the desired wavelength (also known
as the target wavelength), λ.

[0031] Using a second approach, if the optical rotary dispersion of the
sample 24 is known or can be approximated, then the measured light beam
polarization rotation may be corrected using the wavelength error.

[0032] Using a third approach, if the adjustment resolution of the
wavelength selector 9 is sufficiently fine, the wavelength selection
setting can be adjusted to cancel the wavelength error calculated in
equation (4), and the rotation of the polarization of the light beam 21
may be measured again using the corrected wavelength, as set by
wavelength selector 9, using the apparatus and methods discussed above.
Once a correction has been determined at a particular wavelength, it can
be also applied to subsequent light beam polarization rotation
measurements, so long as the wavelength selector 9 remains at that
wavelength.

[0033] Embodiments of the present invention preferably reduce or eliminate
errors in measurements of light beam polarization rotation that are
caused by wavelength errors that occur in existing wavelength selectors.
Moreover, any error source may be corrected where there is a known
relationship between the effect on the optically active sample and the
effect on the polarization rotator. For example, for a fixed wavelength
measurement, a known change in the quantity (M2-M1) could be
used as to correct the optical rotation measurement for any error that
occurs in proportion to the value of the wavelength.

[0034] Reference herein to the use of a Faraday-modulated nulling
polarimeter as the polarimeter of choice is for illustrative purposes
only. The novel features disclosed herein may be applied to systems using
other types of polarimeters such as but not limited to: non-modulated
polarimeters; polarimeters with continuously rotating elements;
photoelastic elements; and various other configurations.

[0035]FIG. 3 is a block diagram of a computing system 300 adaptable for
use with one or more embodiments of the present invention. For instance,
variations of computing system 300 may be included within devices in
apparatus 200 such as but not limited to controller 7, signal generator
4, and/or detector 10. Additionally or alternatively, a computer system
300 (which may also be referred to as a processor) may be deployed in
addition to the devices shown in FIG. 2, and may be used to conduct the
computations associated with equation (1) through equation (4), which are
shown earlier in this document. Such a separate computing device 300 is
preferably placed in communication with detector 10 and/or other
components of apparatus 200.

[0037] In an embodiment, RAM 306 and/or ROM 308 may hold user data, system
data, and/or programs. I/O adapter 310 may connect storage devices, such
as hard drive 312, a CD-ROM (not shown), or other mass storage device to
computing system 300. Communications adapter 322 may couple computing
system 300 to a local, wide-area, or global network 324. User interface
adapter 316 may couple user input devices, such as keyboard 326, scanner
328 and/or pointing device 314, to computing system 300. Moreover,
display adapter 318 may be driven by CPU 302 to control the display on
display device 320. CPU 302 may be any general purpose CPU.

[0038] It is noted that the methods and apparatus described thus far
and/or described later in this document may be achieved utilizing any of
the known technologies, such as standard digital circuitry, analog
circuitry, any of the known processors that are operable to execute
software and/or firmware programs, programmable digital devices or
systems, programmable array logic devices, or any combination of the
above. One or more embodiments of the invention may also be embodied in a
software program for storage in a suitable storage medium and execution
by a processing unit.

[0039] Although the invention herein has been described with reference to
particular embodiments, it is to be understood that these embodiments are
merely illustrative of the principles and applications of the present
invention. It is therefore to be understood that numerous modifications
may be made to the illustrative embodiments and that other arrangements
may be devised without departing from the spirit and scope of the present
invention as defined by the appended claims.